Computers use many different microchips and processors, with perhaps the most
familiar being the CPU and the main memorywhat people commonly refer to as
the computer's RAM. Memory is just a temporary place to store information
until the CPU can get to it. This information can be program instructions, data,
or both. A typical instruction might be a request to store a number or an event
somewhere. Another might be to retrieve that information from a particular
placean address. Volatile memory can only hold information
when a normal electrical current is present. Nonvolatile memory can hold
information without any electrical current.

CAUTION

Volatile, from the Latin "to fly," means that information
"flies away" when there's no electricity to keep it in place.
Television reporters often refer to an explosive situation as a volatile
situation, meaning that it could change at any second. Nonvolatile memory,
because it is not volatile, stays the same without any need for electricity.

Conceptual Overview

Memory is fairly easy to understand, once you've grasped the basic
concepts. In a nutshell, a CPU moves bits of data into registers (storage places
inside a chip). After it's dealt with these data bits to its satisfaction,
the CPU works together with a memory controller to move the results out to
memory cells (storage places on a memory chip). Both registers and memory cells
have memory addresses, and every time a bit of data goes somewhere, it crosses a
bus of some kind. That's it! Now go pass the exam.

All right, so it's a bit, so to speak, more complicated than that. Most
memory began as dynamic random access memory (DRAM), and the main changes have
been to either speed up the memory to match the CPU, or to speed up the CPU to
match the memory. For the most part, the history of memory development revolves
around synchronizing these two subsystems.

NOTE

When we refer to speeding up memory, this usually means increasing either the
actual speed of the chips or increasing the clock speed of associated buses.

Memory involves several basic concepts, the first of which is a grid or
matrix. Because of this, we're going to put Table 3.1 to a slightly
different use, making it a sort of "mind map." If you can see the way
the overall concepts break down on a grid, then perhaps they'll be easier
to remember.

NOTE

A matrix is nothing more than an arrangement of columns and rows, like a
spreadsheet or an Etch-a-Sketch. Columns go across the page, and rows go down
the side. Cells going left to right (horizontally) have an X coordinate. Cells
going up and down (vertically) have a Y coordinate. The direction of rows or
columns is called the axis. Combining both the X and Y coordinates gives us an
address in the grid, like a cell address in a spreadsheet.